Study On The Influence Mechanism Of The Eating Quality Of Rice Based On Structural Analysis And Real-time Monitoring

Eating quality is the most important quality index of rice.The composition characteristics,structural properties,and cooking process are the key factors that affect the eating quality of rice.These factors have been the hot research topics that people are pursuing to improve the eating quality of rice.However,the kernel structure of mature rice is compact and hard to obtain the whole section.The distribution and morphology of the components in rice are still unclear.The researches carrying out on the eating quality of rice are mainly confined to determine the correlation analysis.Meanwhile,studies on the rice cooking are mostly served for the development of rice cooker,but the basic theoretical work has been not enough.Therefore,this thesis takes japonica rice as the object and mechanical and thermodynamic properties as indexes to study the effective mechanisms of the eating quality of rice.In order to figure out the presence and change process of each component in rice kernel,the different structure levels of mature rice were comprehensively studied by the structural analysis.The variation regularity of texture changes,water and constituent migration,and structural evolution were analyzed through real-time monitor during the whole cooking process of rice.The mechanism of various factors influencing on the eating quality were explored.The main research contents are as follows:Due to the mature rice kernels are rigid and fragile,the tough kernel structure was improved and the brittleness was reduced by mixing the fixatives and optimizing the embedding process.This made it possible to obtain the semi-thin and ultra-thin sections of mature rice kernel.Furthermore,the stereoscopic sense microscope was used to observe the whole shape,transparency,and apparent structure of rice.After being frozen and quenched,the compact structure and gap distribution in rice were observed by the scanning electron microscope(SEM).The analysis method was established to reveal the different level structure of the mature rice kernel.It was found that the distribution of starch in different parts was different.The rice protein was spherical,and distributed in the gaps between the cell walls and amyloplasts.Cell walls had the spontaneous fluorescence and presented like a honeycomb-like appearance in rice kernel.The bundle-like structure of cell walls tightly wrapped around the starch granules and protein bodies.The principal component analysis(PCA),cluster analysis,and orthogonal partial least squares(OPLSDA)were used to analyze the differences of different rice cooking procedures.The results showed that the initial 1400 s,especially 0 s-700 s,was the characteristic differential stage of different cooking procedures.The wide range and multi-stage rice cooking programs were established,and the thermodynamic and mechanical properties of rice kernels were monitored by differential scanning calorimetry(DSC)and dynamic rheometer.Under rice cooking conditions,the endothermic change of starch in rice kernel showed two stages: one was the transition from the smectic to the isotropic,the other was from the isotropic to the disordered.A dynamic rheometer was used to simulate the different cooking conditions,and the moduli changes of rice kernel were monitored.It was found that there were two phase transformation for rice during cooking,which were glass transition and starch gelatinization,respectively.The degree of phase transformation was closely related to the cooking rate and soaking temperature.The changes of water content,water distribution,and proton mobility during the whole cooking process were measured.It was found that the physical infiltration was the main path for water migration at the initial cooking stage,and the thin gaps among components were the main channels of migration.When the temperature increased to the starch gelatinization temperature,the migration rate increased,starch granules absorbed water and swelled.The cracks formed by phase transformation provided space for further migration of water.At even high temperature of the holding stage,the interaction between water and different components increased,while the mobility of molecules decreased.Thus,a more even distribution of water was shown in rice kernel.Both heating rate and soaking temperature had closely relationship with the water migration and mechanical properties of rice kernel,which showed different migration paths and structure evolution of components in rice.Thus,those factors affected the final viscoelasticity of cooked rice.The relative molecular weight mass and molecular size of macromolecules in the leached solids and rice kernel were analyzed by gel permeation chromatography with different detectors combined.Then the mechanism of leached materials and structural evolution during the rice cooking was explored.It was found that the proportion of starch and protein in leached solids was low at 30°C-60°C,while the proportion of non-starch and non-protein polymers was high.With the temperature increasing from 70°C to 100°C,the proportion of starch and protein increased gradually.During this period,starch mainly leached in the form of starch granules,while amylose was not the main leakage in the leached solids.During rice cooking,the relative molecular mass of polymers in leached solids increased,while it kept stable in the rice kernel.The structural differences of macromolecules at different cooking stages were mainly due to the changes of molecular size,volume,and conformation of starch.Those changes determined the changes of multi-scale structure of starch,and then affect the final eating quality of cooked rice.The leaching behavior of the components in rice began from 70°C,while the change of long scale structure of starch became obvious at about 90°C.The fully disorder of double helix structure occurred after heating at 100 °C for 15 min.The in-situ analysis of the morphological changes of different components in rice kernel was revealed by analyzing the multi-level structures of rice kernel at different cooking temperatures and time intervals.The cell walls of rice kernel,whose toughness was strong,was the skeleton to maintain the kernel structure of rice.During rice cooking progress,the starch granules absorbed water and swelled,leading to the break of cell walls.The cell walls at the layer surface,instead,were dense and thick,which remained stable at high temperatures and restricted the gelatinization behavior of starch in cells.The morphology of rice protein bodies has high thermal stability,while part of its secondary structure changed during cooking.During rice cooking progress,the interaction between starch and cell walls was strong.The cell walls ruptured and intertwined with the starch gels when the starch was gelatinized.The degradation of cell walls had a great influence on the mechanical properties of rice.However,the interaction between starch and protein was weak.In the rice kernel,protein affected the starch gelatinization by steric hindrance,thus affecting the eating quality of cooked rice.In conclusion,this study established the method to observe the different levels of rice kernel and the real-time process monitoring of rice cooking.Then,the dynamic changes of the thermodynamic and mechanical properties,the leaching behavior,and the structure evolution of the components in rice were explored.Meanwhile,the formation and influence mechanisms of eating quality of rice during cooking were expounded.